Background: Although research-to-policy (R2P) fellowships are increasingly used to facilitate mobility, promote knowledge-exchange, and support evidence-based policy making, the evaluation of these initiatives in terms of (multi-level) impacts and broader 'ecosystem effects' remains under-researched. Aims and objectives: The aim of this article is to evaluate the degree to which the first cohort of Economic and Social Research Council (ESRC) Policy Fellows (2021-2023) can be seen as a successful pilot initiative. The broader objective being to make a distinctive and original contribution to the debate about 'what works' when it comes to promoting research-policy engagement and contribute to research impact evaluation methods. Methods: Two rounds of surveys were conducted with fellows and hosts towards the beginning and middle of the scheme. Survey data was then used to design a semi-structured interview framework. Interviews were conducted with 18 fellows and ten representatives from policy host institutions. A thematic analysis of interview transcripts was then conducted using qualitative data analysis software. Findings: The central argument of this article is that although the early-stage evaluative evidence suggests that the ESRC Policy Fellows initiative 'worked' in terms of its primary ambitions, significant questions exist in relation to systemic concerns that may well limit medium- and long-term impact attainment. Discussion and conclusion: As facilitating the mobility of people, knowledge and talent across traditional disciplinary, professional and institutional boundaries continues to form a key driver within the research, development and innovation 'ecosystem', the results of rare cohort evaluation studies such as the one outlined in this article take on added significance. Opportunities exist to refine and align future investments for maximum social and scientific value.